Aneurysms are circumscribed dilations connecting directly with the lumen of a blood vessel. Aneurysms may result from acquired or congenital diseases which create a weakness in the blood vessel wall. It should be appreciated that aneurysms have a relatively turbulent flow of blood through the "neck" of the aneurysm. AVMs are inappropriately interconnected vessels that shunt blood from the arterial circulation to the venous flow. AVM defects are usually congenital. Depending on vessel size and the nature of the interconnections that make up the malformation, AVMs can also be accompanied by substantial blood flow. In both of these types of vascular defect the presence of blood flow complicates treatment.
Typically aneurysms have thin walls which are vulnerable to a sudden rupture. If an aneurysm ruptures, the resulting hemorrhage can cause death. A hemorrhage can also cause both excessive pressure on nearby tissues and a reduction of blood flow to the tissues "downstream" of the aneurysm.
Neurosurgeons have perfected certain conventional treatments for embolizing aneurysms in small vessels of the brain. The embolization process causes the aneurysm to heal in a way that removes the "pocket" from the wall of the vessel. In the most mature technique, the neurosurgeon accesses the region of the aneurysm under direct visualization through a craniotomy and places one or more aneurysm clips on the "neck" or opening of the aneurysm. This conventional open surgical approach has a high rate of success, but is highly invasive and undesirable for that reason. The risk associated with conventional approaches has encouraged the development of minimally invasive intravascular treatment approaches. Currently, the most widely used minimally invasive technique, involves placement of small coils of wire into the aneurysm. The coils are delivered transluminaly through a catheter that is navigated to the aneurysm site. Once the catheter is advanced into the neck of the aneurysm, multiple coils are released into the aneurysm. The coils dramatically reduce the blood flow through the aneurysm, which results in clotting within the aneurysm. The GDC coiling procedure can be time-consuming due to the number of coils that need to be inserted and released to fill or "pack" the aneurysm. The coiling procedures often need to be repeated because it is difficult to completely `pack" the aneurysm with coils. In this procedure the "neck" of the aneurysm helps to retain the coils. Consequently there are a substantial number of aneurysms which cannot be treated using coils because the large neck allows the first coil to escape from the aneurysm. This common problem is seen radiographically as the first coil curving out of the aneurysm and entering the associated vessel branch. Retrieving an repositioning coils is both difficult and time consuming.
In additions to "coils", liquid or semi-liquid occlusive embolic materials have been proposed for use in treating aneurysms. These fluids include iron-acrylic compounds, as well as, a large collection of biocompatible glues and polymers. Attempts have been made to inject the occlusive materials directly into aneurysms to fill the cavity of the aneurysm. Typically this procedure uses a needle to pierce the wall of the aneurysm to place the material. There has been limited success using these fluids due to the tendency of the blood flow to pass the embolic material out of the aneurysm. This "wash out" situation presents serious complications due to the high probably of occluding the distal branches of the cerebral vasculature. Some investigators have attempted to treat aneurysms with liquids that denature proteins such as alcohol and the like. These procedures are risky as well due to the fact that the treatment occurs in a vessel with active blood flow and the denaturing process can occur in healthy tissue by accident.
The literature has reported several studies where magnetic particles are combined with other materials to serve as an embolic material. The studies have reported improved success in treating aneurysms. In one representative study, a needle surrounded by a magnet was inserted through a burr hole in the patient's head. A stereotactic frame attached to the patient's head was used to direct the needle to the dome of the aneurysm. The mixture of magnetic particles and occlusive agent was injected through the needle, across the wall of the vessel. The study revealed several defects or shortcomings including the need for a burr hole in the skull and a prolonged embolization time for the occlusive agent. Complications also resulted from rupturing the aneurysm. In the reported study the magnet remained in place in the patient's head for several days to completely embolize the aneurysm. The reported technique was unable to hold a magnetic mixture in aneurysms greater than about 1-cm in diameter.
AVMs may also be treated with embolic materials. The most frequently reported technique involves the use of very small polyvinyl alcohol particles, which are sized to occlude the smallest fistulas in the AVM. This technique involves "guess work" to prepare the right size particles that will not pass through the AVM. Also, there is limited control of the emboli, since the catheter placement and the direction of greatest blood flow dictate the delivery of the embolic particles. Visualization of the particles is problematic as well.
For these reasons it is desirable to improve treatment techniques for vascular defects such as aneurysms and AVMs.